Rework combineLatest to remove the Sendable constraint on the base iterator (#228)

# Motivation
This follows the recent rework of the `merge` and `zip` algorithms where we dropped the `Sendable` constraint on the `BaseX.AsyncIterator`. This PR now removes the same constraint on `combineLatest`.

# Modification
This PR overhauls the implementation of `combineLatest` with the goal of changing these things:
- Drop the `Sendable` constraint of the base `AsyncIterator`s
- Only create a single `Task`

# Result
`combineLatest` now doesn't require the base `AsyncIterator` to be `Sendable`

Author: FranzBusch